This is brings back very old memories. My first job out of uni was as an SDE at Ericsson and my first job was writing VLR and MSC software for the first GSM networks in the US. My initial project was handling DTMF so if you pressed a number on the phone, my code handed that! Back in the day, prior to the web taking off, many exceptionally talented programmers/hackers worked in telephony. Then the web took off and most left for greener pastures.